1. Field of the Invention
The invention relates to fiber optic distributed sensors and methods of measuring parameters and calibrating parameter measurements made using optical fiber distributed sensors. In particular, methods of measuring temperature and calibrating temperature measurements made using fiber optic distributed temperature sensors are disclosed.
2. Description of Related Art
Optical fibers typically include a core, a concentric cladding surrounding the core, a concentric protective jacket or buffer surrounding the cladding. Generally the core is made of transparent glass or plastic possessing a certain index of refraction and the cladding is made of transparent glass or plastic possessing a different index of refraction. The relative refractive indices of the core and the cladding largely determine the function and performance of the optical fiber. As a beam of light is introduced into the optical fiber, the velocity and direction of the light changes at the interface of media with different refraction indices. The angles of reflection and refraction can be predicted using Snell's law if the refractive indices of both media are known. It is known to alter the media with their respective refraction indices to provide optical fiber with certain light propagating characteristics. Typically, for minimal power loss, it is desirable for the light to propagate mainly through the core of the optical fiber. In addition to refraction indices, other factors that affect the propagation of the light through the fiber optic core include the dimensions of the core and the cladding, the wavelength of the light, the magnetic field vectors of the light and the electrical field vectors of the light, the configuration of the optical fiber, the presence of imperfections, and environmental effects such as bends, twists, creases or folds.
One advantage of optical fiber is the ability to determine information concerning a parameter of interest relating to environmental effects along the length of a fiber. Measurements are made by introducing optical energy into an optical fiber and receiving backscattered light returned from various distances along the optical fiber. In order to relate the characteristics of backscattered light to the parameter of interest at a particular distance, it is known to use optical time domain reflectometry (OTDR) to determine the distance from which the light is returned along the fiber is required. Such methods are known and described in U.S. Pat. No. 4,823,166 to Hartog et al. and U.S. Pat. No. 5,592,282 to Hartog, both of which are incorporated herein in the entirety by reference. In OTDR, a pulse of optical energy is introduced to the optical fiber and the backscattered optical energy returning from the fiber is observed as a function of time, which is proportional to distance along the fiber from which the backscattered light is received. This backscattered light includes the Rayleigh spectrum, the Brillouin spectrum, and the Raman spectrum. The Raman spectrum is the most temperature sensitive with the intensity of the spectrum varying with temperature, although all three types of backscattered light contain temperature information.
Fiber optic (FO) sensors employ the fact that environmental effects can alter the amplitude, phase, frequency, spectral content, or polarization of light propagated through an optical fiber. Fiber optics sensors can be classified as intrinsic or extrinsic. Intrinsic sensors measure ambient environmental effects by relying on the properties of the optical fiber only while extrinsic sensors are coupled to another device to translate environmental effects into changes in the properties of the light in the fiber optic. Intrinsic fiber optic distributed temperature sensors (DTS) are known. One such device is disclosed in U.S. Pat. No. 5,825,804 to Sai, incorporated herein in its entirety by reference. Such sensors may be multimode fiber (MMF) or single mode fiber (SMF). Single mode optical fibers have a relatively small diameter and support only one spatial mode of propagation. Multimode fibers have a core with a relatively large diameter and permit non-axial rays or modes to propagate through the core.
Typically the Raman spectrum is used to measure temperature, the temperature distribution is calculated based on the ratio of between the Stokes component and the anti-Stokes component of the Raman spectrum of the backscattered light as follows:
                                          I            as                                I            s                          =                  β          ⁢                                          ⁢                      exp            ⁡                          [                                                                    -                    hc                                    ⁢                                                                          ⁢                  υ                                kT                            ]                                                          (        1        )            where β is a coefficient, h is Plank's constant, υ is Raman shifted wavelength number, k is Bolzmann constant, T is absolute temperature, Ias is the anti-Stokes component and Is is the Stokes component. The Stokes/anti-Stokes ratio
      I    s        I    as  is designated as SAR. Using Equation 1, the temperature at the position along the optical fiber from which the backscattering occurred can be determined.
To measure temperature along a fiber optic (FO) distributed sensor, optical energy is introduced into the fiber and backscattered light is excited. The backscattered signal (light) contains information relating to the point along the fiber from which the backscattering occurred. This light is sensed and processed as a time-sequence signal. A one-dimensional temperature distribution along the optical fiber is thus measured. Within the backscattered light, typically the Raman spectrum is transferred by an optical directional coupler to a measuring apparatus, whereby the Stokes light and the anti-Stokes light in the Raman backscattered light are separated by a filter, detected, and converted to electrical signals in proportion to their associated amplitudes by respective photo-electric converters. It is known to calculate temperature distribution based on the ratio between these components of backscattered light, or alternatively based on measurement of only one component of the Raman spectrum of backscattered light.
In optical fiber, there are losses that can affect backscattered Stokes and anti-Stokes wavelengths differently. For example, the optical energy introduced into the optical fiber naturally undergoes attenuation during transmission through the fiber. Also there may losses owing to environmental stresses like bends or connections. These losses subtract differently from the measured backscattered Stokes and anti-Stokes intensities. These differences in fiber attenuation between Stokes and anti-Stokes wavelengths must be addressed to avoid error in the measured parameter along the FO distributed sensor.
Parameter measurements obtained using a FO distributed sensor comprise the true parameter measurement and a measurement error caused by deleterious influences on the fiber optic distributed sensor. By way of example but not limitation, such deleterious influences can include energy losses due to splices or bends, strains in the fiber, changes in attenuation resulting from aging or environmental conditions, drift in measurements over time, hydrogen ingression, or environmental conditions. Such error is cumulative with distance along a fiber. While certain measurement errors can be predicted based on manufacturer or material calibration information, baseline testing, or tracking of known elements such as splice location, the occurrence and effect of other deleterious influences and the measurement error they introduce is difficult to assess. It is known to deploy an optical fiber in a borehole to obtain distributed measurements of borehole parameters and it can be appreciated that accounting for these deleterious influences and their associated measurement error is particularly difficult when the fiber optic distributed sensor is deployed in a borehole. A need exists for a method of calibrating FO distributed sensors and a particular need exists for a method of calibrating optical fibers deployed in a borehole for use in distributed temperature measurements.
One method for correction is presented in U.S. Pat. No. 5,102,232 issued to Tanabe et al. However this method requires maintaining an optical fiber temperature reference point at a known temperature. Maintaining such a reference point may not be feasible. For example, in downhole application where an optical fiber is disposed in a borehole, it may not be possible to maintain a reference point at a known temperature.
Robust methods for accurately determining parameters using a FO distributed sensor measurements in a borehole are needed. The accuracy of parameter measurements can be limited by the algorithm or methodology used to account for variations in the measurements and such limitations in methodology can exist regardless of whether an optical fiber is deployed in a borehole in a linear or loop configuration. Methods of calibrating parameter measurements obtained using a FO distributed sensor are useful. Methods of measuring a parameter obtained using a FO distributed sensor that include calibration of the parameter measurement are also useful. A particular need exists for methods of calibrating temperature measurements obtained using a fiber optic distributed temperature sensor (FO-DTS) and methods of measuring temperature using a FO-DTS that include calibration of the measurements.